Do We Sense the Rotation on a Rapidly Spinning Planet?

[somega]

It's often said that you don't feel Earth rotation because the gravity acts against the centrifugal force.

Of course this is true but also your body is turned around once each 24 hours.

So I wonder on a planet which is rotating once each 3 seconds and has same g=9,81:

Would you feel the rotation?

 

[phinds]

Of course this is true but also your body is turned around once each 24 hours.

Huh?

So I wonder on a planet which is rotating once each 3 seconds and has same g=9,81, would you feel the rotation?

You'd get blown off the planet immediately, or if it had no atmosphere, you'd die from lack of oxygen. Or something.

 

[somega]

You'd get blown off the planet immediately

I think if g=9,81 you cannot get blown off? g is not the gravity but instead the difference between gravity and centrifugal force.

 

[russ_watters]

It's often said that you don't feel Earth rotation because the gravity acts against the centrifugal force.

Of course this is true but also your body is turned around once each 24 hours.

So I wonder on a planet which is rotating once each 3 seconds and has same g=9,81:

Would you feel the rotation?

If you are asking if you would feel the rotation itself (as opposed to the negative gravity crushing you against the ceiling), no. Why would you think you could?

 

[somega]

Look at the picture. Before the rotation you are looking at a fixed star.
After 180° you are looking in the opposite direction.

Isn't one planet rotation like one rotation on a office chair?

 

[Vanadium 50]

This is getting goofy. Of yourse you can see that the Earth rotates. But that's not what you asked.

 

[DrStupid]

So I wonder on a planet which is rotating once each 3 seconds and has same g=9,81: Would you feel the rotation?

The centrifugal force would result in a tidal acceleration of around 7.9 m/s² between head and foots (assuming a body height of 1.8 m) and the Coriolis force would make you sick when you move your head. Of course you would feel that.

 

[phinds]

The centrifugal force would result in a tidal acceleration of around 7.9 m/s² between head and foots (assuming a body height of 1.8 m) and the Coriolis force would make you sick when you move your head. Of course you would feel that.

You are assuming that 9.81g would keep you on the ground under such a fantastic rotation. How do you arrive at that conclusion?

 

[russ_watters]

Look at the picture. Before the rotation you are looking at a fixed star.
After 180° you are looking in the opposite direction.

Yes, we know what rotation is.

Isn't one planet rotation like one rotation on a office chair?

In some ways, yes. In what way are you referring to? What exactly is it you think you would feel? Please answer - I asked this before. We can't tell you if your understanding is right or wrong unless you tell us what your understanding is!

 

[somega]

You are assuming that 9.81g would keep you on the ground under such a fantastic rotation. How do you arrive at that conclusion?

Isn't 9.81g the difference between the gravity force and the centrifugal force? So on a 9.81g planet you can walk like on Earth regardless of its rotation? (That's what I've read on wikipedia.)Here is another drawing:



On a not-rotating planet you are never accelerated.

On a rotating planet you are accelerated from one direction into the other in 1/2 rotation.

Why should it not be possible to feel this?

 

[russ_watters]

Isn't 9.81g the difference between the gravity force and the centrifugal force? So on a 9.81g planet you can walk like on Earth regardless of its rotation?

Oh - apparent g. You didn't say that before. Assuming such a planet is possible, sure.

Why should it not be possible to feel this?

Again: feel what?

Since you won't tell us, and this is tiresome, I'll guess: you might mean feel dizzy. The answer is no: you don't feel dizzy when rotating, only when you suddenly start or stop rotating.

 

[somega]

Let's assume you have a acceleration sensor (as there is in smartphones).
Of course you need high precision.

You start the measurement at midnight somewhere on the equator.
After 12 hours the Earth rotation has accelerated you and you are now traveling with the same speed but in the opposite direction (see my previous drawing).

The sensor has recorded this acceleration regardless of what caused it.

If the sensor can feel this why should the human body not be able to feel it?
Of course the Earth rotates very slow. But on a planet rotating once every 3 seconds?

 

[Vanadium 50]

If the sensor can feel this why should the human body not be able to feel it?

Now you're just getting silly. A sensor can see infrared radiation. hy can't e? A sensor can detect radio waves. hy can't we? And so on and so on and so on.

 

[russ_watters]

Let's assume you have a acceleration sensor (as there is in smartphones).
Of course you need high precision.

You start the measurement at midnight somewhere on the equator.
After 12 hours the Earth rotation has accelerated you and you are now traveling with the same speed but in the opposite direction (see my previous drawing).

The sensor has recorded this acceleration regardless of what caused it.

If the sensor can feel this why should the human body not be able to feel it?
Of course the Earth rotates very slow. But on a planet rotating once every 3 seconds?

[sigh]
So you really are talking about the "g-force". This is why I hate guessing.

You've defined the problem so the gravitational acceleration is equal in both cases (9.81m/s^2), so there is no difference to feel. If you stand on a scale it will read the same as on Earth because you've defined the problem to make it true.

 

[russ_watters]

Now you're just getting silly. A sensor can see infrared radiation. hy can't e? A sensor can detect radio waves. hy can't we? And so on and so on and so on.

We can detect (not see) infrared and some radio and relevand here, we can feel proper acceleration, so I don't see your point -- this doesn't seem helpful.

 

[somega]

No I'm not talking about g-force! I wrote g=9.81. I thought because of the value it should be clear what I mean.

I've looked up Earth rotation speed. It's 464 m/s.

So when you are now traveling with 464 m/s then after 12 hours you are traveling at -464 m/s.
That's (464*2)/12h m/s = 0.02 m/s²

So in 10 seconds it's an acceleration of 0.75 km/h.

I think I'm right and the Earth rotation is simply too slow so you don't feel the force.

 

[Vanadium 50]

My point is that if the question - which we are all still guessing at - is why an instrument can detect something we can't feel, there are many examples of that. We also don't need to go to this contrived situation.

 

[DrStupid]

You are assuming that 9.81g would keep you on the ground under such a fantastic rotation. How do you arrive at that conclusion?

Even if the apparent gravity of 9.81 m/s² (see #3) is given for the ground, there would still be 1.9 m/s² left on the head. However, that doesn't matter. The OP asked, if he would feel the rotation - not if such a rotation is possible on a planet.

 

[russ_watters]

No I'm not talking about g-force! I wrote g=9.81. I thought because of the value it should be clear what I mean.

9.81 is gravitational acceleration on Earth. Please be clear: in your scenario, does your accelerometer read 9.81 or something different? If something different, then what does it read?

So in 10 seconds it's an acceleration of 0.75 km/h.

That's change in velocity, not acceleration. Acceleration is change in velocity per unit time. We feel acceleration, not change in velocity.

I think I'm right and the Earth rotation is simply too slow so you don't feel the force.

Tough to say at this point, but if you answer my question above, we'll have the answer: if the acceleration is 9.81m/s^2, it's the same as on Earth and there is nothing to feel. If it is something different, then there is something to feel.

 

[russ_watters]

My point is that if the question - which we are all still guessing at - is why an instrument can detect something we can't feel, there are many examples of that. We also don't need to go to this contrived situation.

...but my point was that your point is clearly wrong/irrelevant to the thread, right? We can feel acceleration, and do all the time.

 

[Vanadium 50]

I don't think it's "clearly wrong". Is he talking about human vs. sensor sensitivity? Or something else?

 

[ZapperZ]

No I'm not talking about g-force! I wrote g=9.81. I thought because of the value it should be clear what I mean.

I've looked up Earth rotation speed. It's 464 m/s.

So when you are now traveling with 464 m/s then after 12 hours you are traveling at -464 m/s.
That's (464*2)/12h m/s = 0.02 m/s²

So in 10 seconds it's an acceleration of 0.75 km/h.

I think I'm right and the Earth rotation is simply too slow so you don't feel the force.

This is some kooky kinematics here that you are using. The "acceleration" is directed toward the center of the circular motion. This calculation is faulty.

Secondly, this "earth rotation speed" is vague. On what latitude is this? This speed is not a constant at all location! Someone at the pole of the rotational axis will have zero speed.

There are very serious errors in understanding of basic kinematics here.

And I too want to know what "feel" means here.

Zz.

 

[wrobel]

https://en.m.wikipedia.org/wiki/Foucault_pendulum

 

[ZapperZ]

https://en.m.wikipedia.org/wiki/Foucault_pendulum

I would have already suggested that, but we don't know if being able to DETECT the spinning motion of the Earth counts as "feel" for the OP.

That is why I'm seeking clarification of this unscientific and vague term.

Zz.

 

[Nugatory]

Why should it not be possible to feel this?

The effect is there but it's too small for us to feel. Something similar happens when climb to the top of a tree - the force of gravity on us is very slightly different but not enough for us to notice. Sufficiently sensitive instruments can detect it however, and likewise a device like the Foucault pendulum that @wrobel mentioned above will detect the rotation of the earth

 

[somega]

I think I was wrong.

I forgot that if the speed remains the same and the direction changes then there's no force.

And so there's nothing to feel!

 

[ZapperZ]

I think I was wrong.

I forgot that if the speed remains the same and the direction changes then there's no force.

And so there's nothing to feel!

This is wrong. If the speed remains the same but direction changes, there IS a force. Central forces are exactly THAT! Look up "centripetal force".

Zz.

 

[somega]

https://en.m.wikipedia.org/wiki/Foucault_pendulum

Thank you, this is exactly what I meant!

This effect is often not mentioned when it comes to centripetal/centrifugal forces.

I know on Earth this effect is very small.

That's why I talked about a planet rotating once each 3 seconds (and an acceleration of 9.81 m/s² for falling apples near the surface (I think this is clear enough?)).

 

[ZapperZ]

Thank you, this is exactly what I meant!

What you meant was not what you said. We can't read mind (it is after all, a physics forum, not a psychic forum).

This effect is often not mentioned when it comes to centripetal/centrifugal forces.

This has nothing to do with centripetal/centrifugal force. It has something to do with rotational motion and conservation laws.

I know on Earth this effect is very small.

That's why I talked about a planet rotating once each 3 seconds (and an acceleration of 9.81 m/s² for falling apples near the surface (I think this is clear enough?)).

9.8 m/s² will occur whether the Earth is spinning or not. That is the acceleration due to gravity, i.e. the amount of mass underneath us, not due to any motion.

Zz.

 

[jbriggs444]

As I understand it, we are to imagine a neutronium Earth with a radius of 6000 km, a surface gravity of some 90 million g's and a rotation period of three seconds so that the centripetal acceleration at the equator is one g less than that.

We have a person standing stationary on the surface of this Earth at the equator. Nominally he should be experiencing 1 g of apparent weight. Could he sense the rotation?

Yes, he could. If he nods his head, he will feel the dizziness due to Coriolis forces acting on the fluid in the semi-circular canals in his ears.